The invention relates to a process for producing a thermally loaded casting of a thermal turbomachine.
It has long been known to provide parts of thermal turbomachines which are exposed to hot gas, e.g., turbine blades of gas turbines, with cooling-air bores or with cooling structures, in order to increase the temperature of the hot gas and to extend the service life of the parts in question. The inner side of a cooling system which is of double-walled design and is used for a turbine blade, is cooled by cooling air as a result of the heat being dissipated to the outside. The outer side of the blade is cooled by a film which forms on the surface of the turbine blade. The aim is to make the film cooling as effective as possible and, at the same time, to reduce the amount of cooling air.
Gas turbine blades which operate with film cooling are known, for example, from the publications DE 43 28 401 and U.S. Pat. No. 4, 653, 983.
Furthermore, it is known to use metal felts in turbine blades. This is disclosed, for example, in document DE-C2-32 03 869 or in DE-C2-32 35 230. This use of a metal felt has the task of providing a (internal) cooling system. At the same time, this metal felt can serve as protection against abrasion from external mechanical loads, in particular if it has been arranged on the outer side of the turbine blade and has been coated with a ceramic protective layer. A turbine blade with similar properties is also known from European Document EP-B1-132 667.
However, a less advantageous feature of these blades is that they do not comprise a single part, but rather the metal felt always has to be fitted or applied in a further process step.
The invention is based on the object of providing a process for producing a thermally loaded casting of a thermal turbomachine with an integrated cooling structure which increases the efficiency of the turbomachine. The cooling structure is of the same material as the casting and it is also possible to produce it in a step which is part of the casting process.
According to the invention, the object is achieved by a process in which a wax model of the part to be cooled is prepared, at least one polymer foam is prepared, which is fixed to the wax model or is introduced into a cavity in the wax model, the at least one polymer foam and the wax model are immersed in a ceramic material, the ceramic material accumulating around the wax model and the polymer foam also being filled with the ceramic material, the ceramic material is dried, so that a casting mold is formed, the wax and the at least one polymer foam are removed by a heat treatment, the casting is produced using the casting mold by a known casting process, and the ceramic material is removed.
In a second embodiment, the object is achieved in a similar way. As a distinguishing factor, however, a ceramic insert is prefabricated from a polymer foam with an open-cell structure. This ceramic insert is attached to the wax model or is introduced into a cavity in the wax model and the casting mold is produced as described above.
To maintain the external mass of the cooling structure, it is advantageously conceivable to use a prefabricated mold in which the polymer foam is foamed. The slurry can be applied to the polymer foam when it is still in the mold. In this way, it is even possible to form complicated three-dimensional forms of the cooling structure. For better drying of the slurry which is still liquid, the material of this mold may also contain a binder.
A prefabricated ceramic insert of this type can be heated considerably before being used for production of the casting mold, in order to achieve a particular strength. It is also conceivable to burn out the polymer foam of the insert prior to application to the wax model.
In a third embodiment, the object according to the invention is achieved by separate production of the casting and the open-cell cooling structure. In a further process step, the two parts are joined to one another by soldering or welding.
Furthermore, it is possible for an open-cell cooling structure which faces outward to be covered with a ceramic protective layer, in order to protect the casting from additional external abrasion and from the hot gases which surround it. Because of the open-cell structure of the metal foam, the ceramic protective layer adheres very well thereto and the possibility of flaking caused by the extreme operating conditions is reduced. In addition, cooling below the ceramic protective layer is still ensured provided that the ceramic protective layer does not penetrate all the way through the cooling structure.
In all the abovementioned embodiments it is advantageously possible to use a polymer foam of variable cell size to cool certain regions of the cooling system to a greater or lesser extent than other regions. The process will advantageously be a casting process for producing a single-crystal or directionally solidified component. The thermally loaded casting may, for example, be a guide vane or a rotor blade, a heat-accumulation segment, a platform for the guide vane or the rotor blade or a combustion-chamber wall of a gas turbine or a rotor blade of a compressor.